Thermal spraying, also known as flame spraying, involves the heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. A thermal spray gun is used for the purpose of both heating and propelling the particles.
In one type of such gun (e.g. U.S. Pat. No. 2,961,335, Shepard) the material is fed into a heating zone in the form of a heat fusible powder, generally in a size between about 5 and 150 microns. In another type a rod or wire is fed such as described in U.S. Pat. No. 3,148,818 (Charlop). The heating zone is formed by a flame of some type, such as a combustion flame where it is melted or at least heat-softened. A melted wire tip is atomized by an atomizing blast gas such as compressed air, and thence propelled in finely divided form onto the surface to be coated. The spray head includes a nozzle and a gas cap for providing an annular flame around an axially fed spray material.
Ordinarily a thermal spray gun has a spray head including the nozzle and gas cap mounted directly on a gun body for spraying in a forward direction, for example for coating a flat or external cylindrical surface. However, some applications involve spraying into restricted areas such as the inside of bore holes, for example cylinder bores of pumps or combustion engines. In such cases it is necessary to use an extension for the spray head adapted to deflect or otherwise direct the spray stream transversely so as to coat a side wall. Examples of extensions for wire thermal spray guns are disclosed in U.S. Pat. Nos. 3,122,321 (Wilson), 3,136,484 (Dittrich), 3,056,558 (Gilliland et al) and 3,085,750 (Kenshol). It may be seen that there are several basic types: one uses a blast gas for deflecting the spray stream, another has an angular gas cap to deflect the spray, and yet another combines these two.
In some circumstances there is a tendency for spray material from the wire tip to build up inside of the gas cap and/or on the nozzle face. This can occur in an ordinary straight-spraying gun, but particularly may occur with an extension in which the spray stream is deflected by an angular gas cap, as there is more enclosure of the spray in the gas cap. Also, the typically constricted spray region in a bore hole raises the temperature of the spray head, encouraging adhesion, and causes back deflection of spray particles.
A specific material with a buildup problem in the nozzle is molybdenum spray wire, with which oxidation has caused jamming in the nozzle, a condition to which U.S. Pat. No. 2,960,274 (Shepard) is directed by providing a wire guide insert in the nozzle. Buildup is also associated with starting and stopping of spraying, as in repetitive operations. A bulge or "mushroom" may develop on the wire tip under ordinary stopping conditions, which may jam or spit off and stick to the gas cap upon subsequent startup.
As generally shown in the aforementioned patents, a spray wire is driven by an electric motor or air-driven turbine. Further details of mechanisms including drive rolls for gripping and feeding the wire are illustrated in the aforementioned U.S. Pat. No. 3,148,818. As also pointed out in U.S. Pat. Nos. 2,150,949 (Stevens) and 3,378,203 (Stanton), the conventional practice is to coordinate starting and stopping of wire feed with simultaneous changing of gas flows.